1. Field of the Invention
This invention relates to an improved method for preparing radio-frequency (“rf”) accelerator cavities, and more particularly, this invention relates to an improved method for preparing high voltage-gradient superconducting rf cavities through controlled deposition of dielectric and superconducting materials.
2. Background of the Invention
Superconductors are metals that exhibit vanishing resistance at very low temperatures, i.e. temperatures a few degrees above absolute zero (0 degrees Kelvin). Typically, such temperatures are achieved by placing the metal in thermal contact with liquid Helium. Superconducting radio-frequency (“SCRF”) cavities have been adopted throughout the world for the acceleration of particle beams, but SCRF cavities have not been able to routinely reach theoretically expected performance for a variety of reasons. As a result, accelerator designs have been increasing in complexity, cost, and length over the past decades, due to the inability to increase the accelerating voltage gradient of superconducting cavities over 30 MV/m (Million Volts/meter). The International Linear Collider (ILC) design, which is currently estimated to be about 20-25 miles long, is based on the limits of today's technology.
As shown in FIG. 1, a typical SCRF cavity designated as numeral 10 resembles a bellows conduit having alternations of constrictions 15 and recesses 19. An rf voltage generator induces an electric field inside the cavity such that electrons injected at an end 16 of the cavity are accelerated towards the other end 17. Typically the generator frequency is 1.3 GHz. Fabricated from heavy sheet metals, SCRF cavities have many surface imperfections and oxide layers, even after extensive cleaning procedures. As the rf-generated current is confined within roughly the first 100 nanometers of the cavity's surface, the main sources of cavity failure are these surface imperfections. Imperfections include protrusions (referred to herein as asperities), burrs, ridges, scratches, and other defects.
The inventors have found that high local fields result from small local radii of the imperfections. Where local radii of the asperities are larger, the local fields are smaller, and the cavities do not fail. Approximately, the local field E is inversely proportional to the local radius, thus E˜1/r.
The largest electric fields in a SCRF cavity are found near the constrictions 15 and the largest magnetic fields have circular field lines at the recesses 19 in planes perpendicular to the cavity axis α.
A SCRF cavity is first formed from superconducting material, such as niobium. Then, the inside of the structure is electropolished, cleaned and treated in a variety of ways. Finally, the structure is rinsed with high-pressure water. The structures so fabricated accommodate a maximum field of 30 MV/m at 1.3 GHz.
These and other typical superconducting structures fail as a result of a number of mechanisms, for example: 1) field emission (in which free electrons circulating in the highly conductive metal get pulled out of the metal's surface, generating discharges that can “short out” or overheat the structure), 2) quench fields, where the magnetic field exceeds the maximum field that the superconductor can support, 3) high field Q slope, where losses (due to magnetic oxides) degrade the ability of the cavity to store energy, 4) “multipactor”, where resonant amplification of parasitic currents is caused by surface properties, 5) thermal effects (circulating current-heating of the material, thus causing stresses and deformation), 6) breakdown, where arcs are produced, 7) power and cryogenic load limits, 8) assembly defects and particulate generation, 9) Lorentz forces, where internal fields distort the structure, 10) microphonics, where external acoustic noise distorts the structure, and 11) local heating of hot spots.
Cavities fabricated from ordinary metals, such as copper, silver, stainless steel, can also fail at comparable gradients.
It is important that all of the above failure mechanisms be addressed in the search for higher gradient SCRF cavities.
A need exists in the art for a method to increase the energy gradient of SCRF accelerator cavities over the current limit of 30 MV/m. The method should provide economic benefits by making the ILC and other accelerators shorter, more power efficient, capable of reaching higher energies and by reducing construction and operating costs. The method should also enable the production of “table top” accelerators and smaller, high-gradient cavities compared to the size of cavities now used.